This invention relates to methods and apparatus for reducing skin friction drag and to systems utilising such methods and apparatus, particularly in the field of reducing the drag force on high velocity vehicles passing through the atmosphere.
The invention is particularly suitable for, but not limited to, the reduction of drag in vehicles travelling at high speeds through the atmosphere. Of particular interest are vehicles travelling at hypersonic speeds, i.e. travelling at speeds of about or greater than five times the speed of sound in the atmosphere. However, vehicle travelling at near hypersonic speeds (e.g. between Mach 4 and 5) and at supersonic speeds are also of interest and the invention relates also to such vehicles. When used in this specification, the term xe2x80x9cvehiclexe2x80x9d is intended to include for example aircraft, space planes, missiles, projectiles and other such flying objects.
Presently used propulsion systems for launching satellites into earth orbit use rocket propulsion systems in which the fuel and oxidiser are carried by the vehicle. There have been proposals and experiments to develop vehicles for atmospheric travel, including satellite launching, using hypersonic air breathing propulsion systems. These propulsion systems extract the required oxygen for combustion from the atmosphere instead of the vehicle carrying the oxidiser. One of these alternative propulsion systems is known as a supersonic combustion ramjet or xe2x80x9cscramjetxe2x80x9d. This proposed system creates the thrust through heat release by combustion in supersonic flows.
There are two major contributions to drag of vehicles in flight, namely a drag due to the pressure force acting on the surface of the vehicle and skin friction which is the shear force acting tangentially upon the surface of the vehicle due to the passage of air over the surface. Skin friction becomes an increasingly significant part of the drag as the speed of the vehicle increases and, at hypersonic speeds, skin friction can become the dominant component of the drag. The skin friction drag includes not only such drag generated on the outside surfaces of the vehicle but also drag on the inside surfaces of the engine, e.g. in the combustion chamber of a scramjet. The drag in the combustion chamber can contribute in the order of 30% of the overall drag.
It is known that the skin friction drag is due to the viscosity of air, which causes retardation of that part of the air stream passing around the vehicle and which is adjacent to the surface of the vehicle including both external surfaces of the vehicle and internal surfaces within the scramjet, particularly in the combustion chamber. The part of the air stream which is retarded by viscous effects is referred to as the xe2x80x9cboundary layerxe2x80x9d. It has been proposed in the past to cool the combustion chamber walls and to reduce skin friction by introducing a flow of a gas, such as chemically inert helium, into the boundary layer, which is a technique known as xe2x80x9cfilm coolingxe2x80x9d.
However after several decades of theoretical and experimental work, the practical feasibility of scramjet propulsion systems for vehicles travelling at hypersonic speeds is still significantly constrained by limitations caused by skin friction drag.
It is an object of the present invention to provide a method and apparatus for reducing the skin friction drag in an object travelling at high speed relative to a passing fluid.
It is a further object to provide operational objects or systems such as propulsion systems or high speed vehicles, utilising the methods and apparatus of the present invention to reduce skin friction drag.
According to the present invention there is provided a method of reducing the skin friction drag acting on a surface of an article travelling at high speed relative to a passing fluid, the method including the steps of introducing a fuel into the boundary layer of the fluid relatively passing along the surface of the article at high speed, and providing conditions of the fuel introduction to ensure that the introduced fuel undergoes combustion in the boundary layer and while relatively passing the surface of the article downstream of the introduction thereof.
The introduction of fuel in a manner to undergo combustion in the boundary layer has been surprisingly found to reduce skin friction by an extent substantially greater than predicted from inert gas film cooling predictions or analyses.
The fuel may be a gas, vapour, liquid or particulate aerosol or solid, or a mixture thereof.
Preferably the fuel is introduced into the boundary layer in a manner which ensures that a significant quantity will remain in the boundary layer sufficient to support vigorous combustion therein.
It is believed that it is preferable to ensure the fuel remains in and combustion occurs in the boundary layer. The fuel may be injected substantially parallel to the local flow direction of the fluid relatively passing the article. For example, the fuel may be injected through a slot or through orifices provided in the surface of the article past which the fluid is relatively passing, the slot or orifices being arranged so that the fuel enters the passing fluid with a major component (and most preferably substantially the entire component) of direction of injection being parallel to the local flow direction.
As an alternative to injection of fuel parallel to the local flow directions injection of fuel through a slot or orifices at the surface of the article may be at an angle to the local flow direction so that the fuel is compelled by aerodynamic effects to substantially flow within the boundary layer downstream of the injection zone.
Whether the direction of injection is substantially parallel to the local flow direction or is at an angle to that direction but aerodynamic effects cause the fuel to flow within the boundary layer, the slots or orifices are preferably arranged to inject the fuel at high speed and preferably at supersonic speed (although this injection speed may be substantially less than the relative speed of the passing fluid which be at hypersonic speed). A slot or a number of slots or orifices may be provided in the surface of the article, e.g. in a step or shoulder facing downstream so that the fuel is injected in the desired direction. In the case of a scramjet engine, the fuel may be injected around substantially the entire internal circumference of the outside wall of the scramjet engine, e.g. upstream of the commencement of the combustion chamber and possibly also downstream of the combustion chamber. For example, a continuous substantially annular slot may be provided around the entire internal circumference of the outside wall of the scramjet engine.
As an alternative to injection of fuel through a slot or orifices, there may be provided a porous surface on the article through which the fuel is injected under pressure into the passing fluid whereby the fuel travels along the boundary layer and combustion occurs therein.
As a further alternative possibility, fuel may be introduced into the boundary layer by ablation from the surface along which the fluid is travelling.
There may also be other suitable means for introducing fuel, including use of the methods mentioned above in various combinations.
To ensure that the fuel introduced into the passing fluid undergoes combustion in the boundary layer, the method of the present invention may include controlling thermodynamic parameters of the fuel and/or passing fluid to ensure combustion occurs. In particular, the temperatures and pressures of the fuel and of the passing fluid may affect whether the conditions exist to ensure combustion within the boundary layer. The temperatures and pressures will depend upon the fuel being used. The specific example described later will exemplify such thermodynamic parameters.
Another possibility to ensure combustion in the method and apparatus of the present invention is to introduce fuel additives which will extend the range of temperatures and/or pressures at which combustion will occur within the boundary layer, e.g. by lowering the ignition temperature or increasing the exothermic heat yield of the combustion that does occur. A yet further possibility of ensuring or optimising combustion within the boundary layer is to seed the passing fluid and/or the introduced fuel with, for example, chemically active free radicals which will extend the range of temperatures and pressures at which combustion will occur within the boundary layer.
Although the present invention has been broadly outlined above in relation to a method for reducing skin friction drag, the invention also provides apparatus which includes means for carrying out the steps of the inventive method.
According to the present invention there is provided apparatus for reducing the skin friction drag acting on a surface of an article travelling at high speed relative to a passing fluid, the apparatus including means for introducing a fuel into the boundary layer of the fluid relatively passing along the surface of the article at high speed, and means for providing conditions of the fuel introduction to ensure that the introduced fuel undergoes combustion in the boundary layer and while relatively passing the surface of the article downstream of the introduction thereof.
The means for introducing the fuel into the boundary layer is preferably operative to ensure that most of the fuel remains in the boundary layer and to ensure that combustion of the fuel occurs in the boundary layer. The means for introducing the fuel into the boundary layer may comprise means for injecting the fuel substantially parallel to the direction of flow of the fluid relatively passing the article. The means for introducing the fuel into the boundary layer may comprise at least one orifice provided at the surface of the article past which the fluid relatively passes, the orifice(s) being arranged so that the fuel enters the relatively passing fluid with a major component of the direction of injection being parallel to the local flow direction at the orifice(s).
The present invention also provides operational objects or systems utilising the method and apparatus of the present invention, such as scramjet engines embodying the apparatus of the invention and vehicles incorporating such scramjet engines.
Therefore according to a further aspect of the invention there is provided a scramjet engine having an intake for air, a passage through which the air flows at high speed, the passage having a surrounding wall defining the outside surfaces of the passage, a combustion chamber, and an exhaust for discharge of air and combustion products, the scramjet including means for introducing a fuel into the boundary layer of the air passing along the wall at high speed, the fuel being injected around substantially the entire internal circumference of the wall of the scramjet engine, the scramjet engine further including means for providing conditions of the fuel introduction to ensure that the introduced fuel undergoes combustion in the boundary layer while passing along the wall downstream of the introduction thereof.
The means for introducing the fuel in the scramjet engine preferably injects the fuel substantially upstream of the commencement of the combustion chamber of the scramjet engine. It may be also advantageous to inject fuel along the wall downstream of the combustion chamber.
The means for introducing the fuel may comprise a substantially continuous annular slot provided around the entire internal circumference of the wall. The annular slot may be located in a step facing downstream relative to the passing air so that the fuel is injected in a direction along the wall to ensure the fuel remains in the boundary layer downstream of the step and combustion occurs in the boundary layer downstream of the step.
The present invention also provides a high speed vehicle for travelling through the atmosphere at supersonic or hypersonic speed, the vehicle having a propulsion system including a scramjet engine according to the invention.
It will be convenient to further describe the invention in connection with a particular apparatus developed for experimentally verifying and developing the invention.